Zeolite Active Sites Research Articles

Zeolite-catalyzed Isomerization of 1-Hexene to trans-2-Hexene: An ONIOM Study

Details of the double-bond isomerization of 1-hexene over H-ZSM-5 were clarified using density functional theory. It is found that the reaction proceeds by a mechanism which involves the Bronsted acid part of the zeolite solely. According to this mechanism, 1-hexene is first physically adsorbed on the acidic site, and then, the acidic proton transfers to one carbon atom of the double bond, while the other carbon atom of the double bond bonds with the Bronsted host oxygen, yielding a stable alkoxy intermediate. Thereafter, the Bronsted host oxygen abstracts a hydrogen atom from the C6H13 fragment and the C–O bond is broken, restoring the acidic site and yielding trans-2-hexene. The calculated activation barrier is 12.65 kcal/mol, which is in good agreement with the experimental value. These results well explain the energetic aspects during the course of double-bond isomerization and extend the understanding of the nature of the zeolite active sites.

Zeolites are effective ROS-scavengers in vitro

We report on the use of zeolites to limit the effects of reactive oxygen species (ROS) on human albumin under in vitro conditions. Zeolites of different structure type, channel size, channel polarity, and charge-compensating cation were screened for the elimination of ROS, notably HO , resulting from the Fenton reaction. A test based on ischemia-modified albumin (IMA) was used as a marker to monitor the activity of HO after co-exposure of human serum to these zeolites. Two commercial zeolites, faujasite (FAU 13×, channel opening 0.74 × 0.74 nm with Na + as charge-compensating cation) and ferrierite (FER, channel opening 0.54 × 0.42 nm with H + as charge-compensating cation), were found to reduce IMA formation by more than 65% due to removal of HO relative to reference values. It was established that partial ion exchange of the zeolites’ respective charge-compensating cation vs. Fe 3+ implicated in the Fenton reaction plays a major role in HO deactivation process. Moreover, our results show that no saturation of the respective zeolite active sites occurred. This is possible only when ROS are actively converted to water molecules within the zeolite void system, which generates H + ion transport. Because zeolites cannot be administered in blood, their use in medicine should be limited to extra corporeal circuits. Zeolites could be of use during cardiopulmonary bypass or hemodialysis procedures.

Theoretical Study of the Double-Bond Isomerization of 1-Hexene to <em>cis</em>-2-Hexene over ZSM-5 Zeolite

We investigated the double-bond isomerization reaction of 1-hexene to cis-2-hexene on the surface of ZSM-5 zeolite using density functional theory with a 54T cluster model simulating the local structures of zeolite materials. We found that the double-bond isomerization proceeded by a mechanism that did not involve the bifunctional (acid-base) nature of the zeolite active sites but exclusively involved the acid sites. According to this mechanism, 1-hexene is the first physically adsorbed onto the zeolite acid site resulting in the formation of a π-complex, and then the acidic proton of the zeolite transfers to a carbon atom of the double bond of the physisorbed 1-hexene. The other carbon atom of the double bond of the physisorbed 1-hexene bonds with the host oxygen and yields a stable alkoxy intermediate. Thereafter, the host oxygen abstracts a hydrogen atom from the C6H13 fragment and the C―O bond of the alkoxy intermediate is broken, which restores the zeolite active site and yields physisorbed cis-2-hexene. The proposed reaction pathway competes with the bifunctional pathway. The ratedetermining step is the decomposition of the alkoxy intermediate with an activation energy of 134. 64 kJ· mol. The calculated apparent activation energy for the isomerization reaction is 59. 37 kJ·mol-1, which is in good agreement with the reported experimental value. These results well explain the energetic aspects during the double-bond isomerization and extend the understanding of the nature of zeolite active sites. 1081 Acta Phys. ⁃Chim. Sin. 2011 Vol.27

Effect of Zeolite Chemical Surface Properties on Catalytic Ozonation of Methylene Blue Contaminated Waters

Heterogeneous catalytic ozonation using natural zeolite has been recently reported. However, there is a lack in the information related to the influence of zeolite active surface sites in this combined system. This work presents experimental results on the effect of zeolite chemical surface properties on catalytic ozonation. Zeolite samples with different chemical surface compositions were prepared from natural zeolite. The effect of pH, and the presence of radical scavengers were assessed at laboratory scale. Results obtained here indicate that hydrous oxide sites present on zeolite surface (S–OH2 +, S–OH, S–O−) play a key role on the catalytic ozonation mechanism.

Where Are the Active Sites in Zeolites? Origin of Aluminum Zoning in ZSM-5

We visualized the aluminum zoning in TPA+-templated crystals of ZSM-5 by EDX mapping of cross sections obtained by milling of the crystal with a focused ion beam. Aluminum was preferentially located in the rim, while silicon and oxygen were well distributed. Because of its limited penetration, EDX of the large uncut crystals failed to detect the enrichment. Desilication and dealumination by base and acid leaching supported the results of FIB/EDX mapping, showing that the crystals have a dissolved interior or etched rims. To understand the origin of the heterogeneous distribution of aluminum within the crystal, the location of aluminum and silicon after the synthesis of crystals of TPA+-templated ZSM-5 was determined by analyzing the crystals at different stages of synthesis. The crystals had different sizes, and all showed aluminum zoning. The catalytically active sites are located almost exclusively at the rim of the zeolite crystals. These results have improved our understanding of the mechanism of aluminum incorporation into the ZSM-5 framework. Our results may contribute to the controlled locating of active sites within zeolite crystals.

New zeolite catalyst from Süd-Chemie

Incorporation of niobium into microporous molecular sieves brings up new properties for these materials. ZSM-5 supported catalysts containing 2, 5, 13 and 19 wt.% of niobium pentoxide were prepared by aqueous impregnation and characterized by the combination of thermal (TG/DTA) and spectroscopic methods such as FTIR, FT-Raman, DRIFTS, and 29Si and 27Al MAS and 29Si CP-MAS-NMR. In addition, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and measurements of specific surface area by nitrogen adsorption (BET method) were carried out in order to obtain structural, textural and stability information about the solids. The samples were studied at different temperatures and heating time to optimize these parameters and to avoid a possible reduction of acidic properties by dehydroxylation. MAS-NMR and XRD results showed that the zeolite structure containing Nb did not undergo dealumination after thermal treatment, according to the presence of SiONb units on the zeolites structure in FTIR and Raman studies. Thermal analysis confirmed that Nb2O5 impregnation reduced zeolite ZSM-5 dehydroxylation and thus promoted thermal stabilization. Nitrogen adsorption isotherms by the BET method indicated that surface area and pore volume decreased with the increase of Nb loading on the calcined samples at 450 °C. In addition, the nature of the zeolite active sites was investigated by spectroscopic and thermal analysis after pyridine adsorption. It was proposed that there are two different acidic sites (Brønsted and hydrogen bond site types) on the studied systems.

Use of the IR spectra of adsorbed ethane and propane molecules to characterize the strength of active sites in zeolites and to analyze the activation of C-H bonds in these paraffins

The adsorption and activation of ethane and propane on the hydrogen and cationic forms of mordenite, zeolite ZSM-5, and zeolite Y were studied by diffuse-reflectance IR spectroscopy. The effect of the polarization of these molecules by adsorption sites on the intensities and shifts of absorption bands due to C-H stretching vibrations were studied. It was found that weak adsorption species were formed on the hydrogen forms of the above zeolites. In this case, both the intensity distributions and the positions of absorption bands due to C-H stretching vibrations were almost independent of the nature of zeolites. However, both absorption band maximum positions and relative intensity distributions changed upon paraffin adsorption on the cationic forms. It was also found that relative intensity distributions and shifts of absorption bands due to C-H stretching vibrations strongly depended on the nature of cations and zeolites. In this case, the initially totally symmetrical C-H vibrations were found most strongly disturbed. The low-frequency shifts and relative intensities of absorption bands due to these vibrations for various cations and zeolites were found to increase in the following orders: H < Na < Ca < Mg < Zn and zeolite Y < Mord ≈ ZSM-5. The experimental results suggest that ethane and propane molecules can be used as molecular probes for acquiring information on the nature and properties of acidbase sites in zeolites. In this case, both the low-frequency shifts and the relative intensities of absorption bands due to C-H stretching vibrations can be used as measures to characterize the nature of cations and zeolites. However, the latter was found to be much more sensitive to the nature of active sites.

Effects of niobium addition on ZSM-5 studied by thermal and spectroscopy methods

Incorporation of niobium into microporous molecular sieves brings up new properties for these materials. ZSM-5 supported catalysts containing 2, 5, 13 and 19 wt.% of niobium pentoxide were prepared by aqueous impregnation and characterized by the combination of thermal (TG/DTA) and spectroscopic methods such as FTIR, FT-Raman, DRIFTS, and 29Si and 27Al MAS and 29Si CP-MAS-NMR. In addition, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and measurements of specific surface area by nitrogen adsorption (BET method) were carried out in order to obtain structural, textural and stability information about the solids. The samples were studied at different temperatures and heating time to optimize these parameters and to avoid a possible reduction of acidic properties by dehydroxylation. MAS-NMR and XRD results showed that the zeolite structure containing Nb did not undergo dealumination after thermal treatment, according to the presence of Si O Nb units on the zeolites structure in FTIR and Raman studies. Thermal analysis confirmed that Nb 2O 5 impregnation reduced zeolite ZSM-5 dehydroxylation and thus promoted thermal stabilization. Nitrogen adsorption isotherms by the BET method indicated that surface area and pore volume decreased with the increase of Nb loading on the calcined samples at 450 °C. In addition, the nature of the zeolite active sites was investigated by spectroscopic and thermal analysis after pyridine adsorption. It was proposed that there are two different acidic sites (Brønsted and hydrogen bond site types) on the studied systems.

Effects of crystal size and Si/Al ratio on the surface properties of H-ZSM-5 zeolites

Highly crystalline H-ZSM-5 zeolite samples with crystal sizes from 0.5 to 5 μm and Si/Al ratios ranging from 20 to 70 have been prepared and characterized by IR spectroscopy, SEM and XRD techniques. The surface properties of these materials have been compared with those of conventional small crystal size materials by using FT-IR spectroscopy of the surface hydroxyl groups and of adsorbed carbon monoxide. The external surface has been investigated using pivalonitrile (2,2-dimethyl-propionitrile) as an adsorption probe. Data show that the basic structure of the zeolite active sites do not depend significantly on the crystal size. Brønsted acid sites of high acid strength are constituted by bridging OH groups located inside the zeolite channels, while Lewis acid sites and weakly acidic silanol groups are found on the external surface in all cases.

Formation of Heteroatom Active Sites in Zeolites by Hydrolysis and Inversion

A break and a flip: Quantum mechanical/molecular mechanical calculations are used to investigate the structures of the active sites in silicalite doped with heteroatoms from Groups 4 and 14. These calculations reveal that tripodal (see picture; Si yellow, O red, H white, heteroatom blue) or bipodal sites, formed through hydrolysis and inversion, are preferred.

Formation of Heteroatom Active Sites in Zeolites by Hydrolysis and Inversion

Brechen und Umklappen: Quantenmechanische/molekülmechanische Rechnungen wurden verwendet, um die Strukturen der aktiven Zentren in Silicalit zu untersuchen, der mit Heteroatomen der Gruppen 4 und 14 dotiert ist. Nach diesen Rechnungen sind tripodale (siehe Bild; Si gelb, O rot, H weiß, Heteroatom blau) oder bipodale Zentren, die durch Hydrolyse und Inversion entstehen, bevorzugt.

A DFT study of SN 2 and E 2 reactions of butylhalides on zeolite Y: The effect of the leaving group

A density functional theory (DFT) study of adsorption and reaction of n-butyl, sec-butyl, and tert-butyl halides was carried out at B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d,p) level of calculation, using a NaT 3 (T=Si, Al) cluster to represent the zeolite active site. The results show that the adsorption of the alkyl halides on the zeolite surface involves a charge–dipole interaction and is exothermic with respect to the isolated reactants, especially for butylchlorides. The formation of alkoxy species, through a S N2 type reaction mechanism, is lower in energy than proton elimination, through E2 type reaction mechanism, for the primary and secondary halides. This behavior is more accentuated for butylchlorides than for the other butyl halides. For the reactions with butyl bromides and iodides with NaT 3 cluster, the difference between the energy barriers for alkoxide formation and proton elimination is lower than for the respective chlorides. For the tert-butyl halides, proton elimination was the only reaction found possible in the calculations.

A density-functional theory study on double-bond isomerization of 1-butene to cis-2-butene catalyzed by zeolites

Using density-functional theory, the double-bond isomerization of 1-butene to cis-2-butene over zeolites is investigated with a 3T cluster model simulating zeolite. At the B3LYP/6-31G(d, p) level, the complete geometry optimization and the activation energy calculation are performed. It is found that the OH group of acidic site of zeolite protonates the side C atom of double-bond of 1-butene and simultaneously, the neighboring O atom of the cluster abstracts a hydrogen atom from the butene, restoring the zeolite active site and yielding adsorbed cis-2-butene. The reaction shows a concerted mechanism. The calculated activation barrier is close to the experimental data.

Comparative study of the active sites in zeolites by different probe molecules

This review summarizes some of the recently published results concerning the acid sites in the zeolites ZSM-5 and Y studied by temperature-programmed desorption (TPD) and adsorption calorimetry using different probe molecules NH3, CO, N2O and n-hexane. For the first time it has been shown that the acid sites in hydrated zeolites are accessible for n-hexane adsorption.

DFT Study on Double-bond Isomerization of &lt;i&gt;trans&lt;/i&gt;-2-butene Catalyzed by Zeolites

Using density functional theory(DFT), the double-bond isomerization of 1-butene to trans-2-butene over zeolites is investigated with a 3T cluster model simulating zeolite. At the B3LYP/6-31G(d, p) level, the complete geometry optimization and the activation energy calculation are performed with a E(subscript ZP) correction. It is found that the process include three steps, physical adsorption →chemical reaction →physical desorption. Firstly, the OH group of the acidic site of zeolite adsorbs the double-bond of 1-butene and forms a π-complex. Secondly, the double-bond isomerization reaction shows a concerted mechanism and yields the adsorbed trans-2-butene. Lastly, trans-2-butene is produced by desorption and the zeolite active site is restored. The calculated apparent activation energy is 57.1 kJ•mol^(-1), which is in agreement with the experimental data.

Calculated magnetic properties for the characterization of zeolite active sites.

Detailed structures of zeolite catalysts, including Al and cation distribution, framework and catalytic site geometries, are not fully accessible from experiment. Since the magnetic properties of framework elements and extra-framework cations are strongly dependent on their environment, the combined use of magic angle spinning NMR or ESR techniques with quantum chemical calculations is very useful to establish the local structure around specific sites. General effects of Al and B substitution in the zeolite framework, coupled with H(+) or Na(+) counter-ions, on the (29)Si, (27)Al and (11)B NMR spectra were studied, using a density functional theory (DFT)-based methodology, for a model of the zeolite mazzite. In agreement with experiment, the exchange of Na(+) by H(+) in the boron compound is accompanied by a change of B coordination from tetrahedral to trigonal, with a characteristic downfield shift of around 10 ppm, whereas the presence of water restores the tetrahedral boron and its NMR chemical shift. Further, ESR spectra of zeolites exchanged with open-shell cations provide useful data on the metal coordination and its reactivity compared with calculated model ESR properties. The ESR hyperfine coupling constants, calculated using DFT, for models of different Cu sites of a Cu(II)-Y zeolite, with and without H(2)O or NH(3), show a clear correlation between the Cu spin population and its coordination, involving the participation of the zeolite framework in the reactivity.

Identification of an adsorption complex between an alkane and zeolite active sites.

Direct observation of the Bronsted acid site signal in an active zeolite catalyst following adsorption of stoichiometric quantities of isobutane reveals the presence of a specific adsorption complex. Independent polarization transfer experiments in which magnetization originates with either the catalyst or the adsorbed isobutane confirm this assignment. The initial steps in alkane reactivity are poorly defined, and this experimentally verified complex is proposed as a route to C-H bond activation in solid acids.

Influence of the Counterion on the Local Environment and Electronic Structure of Active Sites in Zeotypes

We investigate, using periodic quantum mechanical calculations, how different counterions affect the geometrical and electronic structure of Al 3 + and Fe 3 + active sites in zeolites and AlPOs. Association of the counterion with one or more framework oxygens modifies their bond lengths within the framework, including the distance from the dopant ion, and hence the symmetry of the active sites. The average bond length of the dopant with its four oxygen neighbors is, however, only marginally affected by the type of counterion. We further find that the presence of framework dopants influences the electronic structure, and in particular that low-valent dopant ions are ionic, while causing an increase of covalence in the surrounding region of the framework.

Microcalorimetry in the Identification and Characterization of the Most Reactive Active Sites of Heterogeneous Catalysts

The microcalorimetric technique has been applied to investigate the strongest active sites of zeolites, heteropolyacids and perovskites. The differential heats of adsorption of carbon monoxide, ammonia and oxygen were determined. The results imply that the method is very useful in the identification and characterization of the most reactive active sites of solid catalytic materials.

SN2, E2 reactions of butylchlorides on NaY zeolite: A potential method for studying the formation and reactivity of alkoxy species on the zeolite surface

The adsorption and reaction of n-butyl, sec-butyl, isobutyl and tert-butyl chlorides over NaY zeolite was studied by infrared spectroscopy. All the chlorides reacted almost instantaneously with the zeolite, giving rise to CH2 and CH3 bands in the infrared spectra. A band at approximately 1640 cm−1, ascribed to CC vibration, was observed in all experiments, its intensity being dependent upon the structure of the chloride. For tert-butyl chloride the intensity is greatest, and bands at 3550 and 3061 cm−1, ascribed to the acidic OH and olefinic CH, respectively, were observed. The intensity of the olefinic band is weaker in secondary and especially primary chlorides. Calculations at the B3LYP/6-31+G** level, using a T3 cluster to represent the zeolite active site, indicated that, for the secondary and primary chlorides, the formation of the alkoxy species, through an SN2 type reaction mechanism, is lower in energy relative to proton elimination, through an E2 type mechanism. The opposite was observed for the tert-butyl chloride, in agreement with the experimental results. The method can be useful for generating alkoxide species, especially primary and secondary ones, and for studying their reactivity on the zeolite surface.